U.S. patent number 4,376,029 [Application Number 06/186,181] was granted by the patent office on 1983-03-08 for titanium diboride-graphite composits.
This patent grant is currently assigned to Great Lakes Carbon Corporation. Invention is credited to Louis A. Joo', Frank E. McCown, Kenneth W. Tucker.
United States Patent |
4,376,029 |
Joo' , et al. |
March 8, 1983 |
Titanium diboride-graphite composits
Abstract
A cathode component for a Hall aluminum cell is economically
produced from a mixture of a carbon source, preferably calcined
petroleum coke, and optionally calcined acicular needle petroleum
coke, calcined anthracite coal; a binder such as pitch including
the various petroleum and coal tar pitches; titanium dioxide,
TiO.sub.2 ; and boric acid, B.sub.2 O.sub.3 or boron carbide,
B.sub.4 C; forming said mixture into shapes and heating to a
TiB.sub.2 -forming temperature.
Inventors: |
Joo'; Louis A. (Johnson City,
TN), Tucker; Kenneth W. (Elizabethton, TN), McCown; Frank
E. (Bristol, TN) |
Assignee: |
Great Lakes Carbon Corporation
(New York, NY)
|
Family
ID: |
22683950 |
Appl.
No.: |
06/186,181 |
Filed: |
September 11, 1980 |
Current U.S.
Class: |
204/294;
501/96.3; 204/291; 252/506; 252/507; 264/29.5; 264/29.7; 423/289;
423/297; 502/101; 502/182; 205/386 |
Current CPC
Class: |
C04B
35/532 (20130101); C25C 3/08 (20130101); C04B
35/52 (20130101); C04B 35/58071 (20130101) |
Current International
Class: |
C25C
3/00 (20060101); C25C 3/08 (20060101); C04B
35/52 (20060101); C04B 35/58 (20060101); C25B
011/04 (); C25C 003/12 () |
Field of
Search: |
;204/291,294,67,29R
;264/29.5,29.7 ;423/289,297 ;252/506,507,425.3 ;501/96 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
528472 |
|
Jun 1955 |
|
IT |
|
1289081 |
|
Sep 1972 |
|
GB |
|
Primary Examiner: Edmundson; F.
Attorney, Agent or Firm: Good; Adrian J.
Claims
We claim:
1. A process for the production of TiB.sub.2 -carbon composite
comprised of the steps of mixing and dispersing particulate
carbonaceous matter and TiB.sub.2 -forming reactants in a
carbon-forming binder to form a plastic mixture, forming a shaped
article from said mixture, baking said article at 700.degree. to
1100.degree. C., impregnating said baked article with a
carbon-forming binder and rebaking said article to 700.degree. to
1100.degree. C.
2. A process for the production of a TiB.sub.2 -carbon composite
article comprised of the steps of mixing and dispersing particulate
carbonaceous matter and TiB.sub.2 -forming reactants in a
carbon-forming binder to form a plastic mixture, forming a shaped
article from said mixture, baking said article to 700.degree. to
1100.degree. C., heating said article to a TiB.sub.2 -forming
temperature from 2000.degree. to 3000.degree. C. wherein the
composite article is impregnated with a carbon-forming binder and
is re-baked to 700.degree. to 1100.degree. C.
3. A process for the production of a TiB.sub.2 -carbon composite
article comprised of the steps of mixing and dispersing particulate
carbonaceous matter and TiB.sub.2 -forming reactants in a
carbon-forming binder to form a plastic mixture, forming a shaped
article from said mixture, baking said article to 700.degree. to
1100.degree. C., further heating said article to a TiB.sub.2
-forming temperature of 2000.degree. to 3000.degree. C. wherein the
composite article is then impregnated with a carbon-forming binder,
re-baked to 700.degree. to 1100.degree. C. and re-heated to
2000.degree. to 3000.degree. C.
Description
DESCRIPTION
BACKGROUND OF THE INVENTION
Aluminum metal has been produced for 90 years in the Hall cell by
electrolysis of alumina in a molten cryolite salt electrolyte bath
operating at temperatures in the range of 900.degree.-1000.degree.
C. The reactivity of the molten cryolite, the need for excellent
electrical conductivity, and cost considerations have limited the
choice of materials for the electrodes and cell walls to the
various allotropic forms of carbon.
Typically the Hall cell is a shallow vessel, with the floor forming
the cathode, the side walls a rammed coke-pitch mixture, and the
anode a block suspended in the bath at an anode-cathode separation
of a few centimeters. The anode is typically formed from a
pitch-calcined petroleum coke blend, prebaked to form a monolithic
block of amorphous carbon. The cathode is typically formed from a
pre-baked pitch-calcined anthracite or coke blend, with
cast-in-place iron over steel bar electrical conductors in grooves
in the bottom side of the cathode.
During operation of the Hall cell, only about 25% of the
electricity consumed is used for the actual reduction of alumina to
aluminum, with approximately 40% of the current consumed by the
voltage drop caused by the resistance of the bath. The
anode-cathode spacing is usually about 4-5 cm., and attempts to
lower this distance result in an electrical discharge from the
cathode to the anode through aluminum droplets.
The molten aluminum is present as a pad in the cell, but is not a
quiescent pool due to the factors of preferential wetting of the
carbon cathode surface by the cryolite melt in relation to the
molten aluminum, causing the aluminum to form droplets, and the
erratic movements of the molten aluminum from the strong
electromagnetic forces generated by the high current density.
The wetting of a solid surface in contact with two immiscible
liquids is a function of the surface free energy of the three
surfaces, in which the carbon cathode is a low energy surface and
consequently is not readily wet by the liquid aluminum. The angle
of a droplet of aluminum at the cryolite-aluminum-carbon junction
is governed by the relationship ##EQU1## where .alpha..sub.12,
.alpha..sub.13, and .alpha..sub.23 are the surface free energies at
the aluminum carbon, cryolite-carbon, and cryolite-aluminum
boundaries, respectively.
If the cathode were a high energy surface, such as would occur if
it were a ceramic instead of carbon, it would have a higher contact
angle and better wettability with the liquid aluminum. This in turn
would tend to smooth out the surface of the liquid aluminum pool
and lessen the possibility of interelectrode discharge allowing the
anode-cathode distance to be lowered and the thermodynamic
efficiency of the cell improved, by decreasing the voltage drop
through the bath.
Typically, amorphous carbon is a low energy surface, but also is
quite durable, lasting for several years duration as a cathode, and
relatively inexpensive. However, a cathode or a TiB.sub.2 stud as a
component of the cathode which has better wettability and would
permit closer anode-cathode spacing could improve the thermodynamic
efficiency and be very cost-effective.
Several workers in the field have developed refractory high free
energy material cathodes. U.S. Pat. No. 2,915,442, Lewis, Dec. 1,
1959, claims a process for production of aluminum using a cathode
consisting of the borides, carbides, and nitrides of Ti, Zr, V, Ta,
Nb, and Hf. U.S. Pat. No. 3,028,324, Ransley, Apr. 3, 1962, claims
a method of producing aluminum using a mixture of TiC and TiB.sub.2
as the cathode. U.S. Pat. No. 3,151,054, Lewis, Sept. 29, 1964,
claims a Hall cell cathode conducting element consisting of one of
the carbides and borides of Ti, Zr, Ta and Nb. U.S. Pat. No.
3,156,639, Kibby, Nov. 10, 1964, claims a cathode for a Hall cell
with a cap of refractory hard metal and discloses TiB.sub.2 as the
material of construction. U.S. Pat. No. 3,314,876, Ransley, Apr.
18, 1967, discloses the use of TiB.sub.2 for use in Hall cell
electrodes. The raw materials must be of high purity particularly
in regard to oxygen content, Col. 1, line 73-Col. 2, line 29; Col.
4, lines 39-50, Col. 8, lines 1-24. U.S. Pat. No. 3,400,061, Lewis,
Sept. 3, 1968 discloses a cathode comprising a refractory hard
metal and carbon, which may be formed in a one-step reaction during
calcination. U.S. Pat. No. 4,071,420, Foster, Jan. 31, 1978,
discloses a cell for the electrolysis of a metal component in a
molten electrolyte using a cathode with refractory hard metal
TiB.sub.2 tubular elements protruding into the electrolyte. The
protruding elements enhance electrical conductivity and form a
partial barrier to the mechanical agitation caused by magnetic
effects.
SUMMARY OF THE INVENTION
Titanium Diboride, TiB.sub.2 has been proposed for use as a cathode
or cathodic element or component in Hall cells for the reduction of
alumina, giving an improved performance over the amorphous carbon
and semi-graphite cathodes presently used.
It had previously been known that Titanium Diboride (TiB.sub.2) was
useful as a cathode in the electrolytic production of aluminum,
when retrofitted in the Hall cell as a replacement for the carbon
or semi-graphite form. The electrical efficiency of the cell was
improved due to better conductivity, due mainly to a closer
anode-cathode spacing; and the corrosion resistance was improved,
probably due to increased hardness, and lower solubility as
compared to the carbon and graphite forms.
The principal deterrent to the use of TiB.sub.2 as a Hall cell
cathode has been the great cost, approximately $25/lb. as compared
to the traditional carbonaceous compositions, which cost about
$0.60/lb., and its sensitivity to thermal shock. If the
anode-cathode distance could be lowered, the % savings in
electricity would be as follows:
______________________________________ A-C distance % savings
______________________________________ 3.8 cm. std. 1.9 cm. 20% 1.3
cm. 27% 1.0 cm. 30% ______________________________________
We have invented an improved process for producing a TiB.sub.2
-carbon composite which shows excellent performance as a cathode or
cathode component in Hall aluminum cells, and which is markedly
more economical. The method also produces an unexpectedly improved
cathode when its performance is compared to the traditional
carbonaceous material.
We have found that our method gives an unexpected advantage in that
the articles produced in this manner are much more resistant to
thermal shock than articles formed by prior art methods using
TiB.sub.2 powder or reactants processed by previously known
methods. In particular, we have found that cathode components for
Hall cells are much more resistant to the severe thermal shock
imposed on them at the temperature of operation in molten
cryolite.
We have also found another unexpected advantage in that we do not
need to use the highly purified raw materials specified in the
previously known methods. We have also used a commercially pure
grade specified to assay at least 98% and typically 99.5% TiB.sub.2
and a grade with 99.9% purity. The various grades are referred to
herein by their nominal purities as given above.
The method involves the use of pre-mixed and pre-milled TiB.sub.2
precursors, i.e., pigment grade titanium dioxide (TiO.sub.2) and
boron oxide (B.sub.2 O.sub.3), or boron carbide (B.sub.4 C) which
are preferably added dry to the coke filler prior to addition of
binder pitch. These reactants are then intimately mixed and well
dispersed in the coke-pitch mixture and firmly bonded into place
during the bake cycle. We have found that the reaction proceeds
well at or above 1700.degree. C., forming the bonded
carbon-TiB.sub.2 composite in situ. Here carbon includes graphite
as well as amorphous carbon.
The normal method of production of monolithic carbonaceous pieces,
either amorphous or graphitic carbon, involves a dry blend of
several different particle sizes of coke and/or anthracite fillers
and coke flour (50%-200 mesh) (79 mesh/cm), followed by a
dispersion of these solid particulates in melted pitch to form a
plastic mass which is then molded or extruded, then baked on a
gradually rising temperature cycle to approximately
700.degree.-1100.degree. C. The bake process drives off the low
boiling molecular species present, then polymerizes and carbonizes
the pitch residue to form a solid binder-coke composite. If the
material is to be graphitized, it is further heated to a
temperature between 2000.degree. C. and 3000.degree. C. in a
graphitizing furnace. A non-acicular or regular petroleum coke or
calcined anthracite may be used to avoid a mismatch of the
Coefficient of Thermal Expansion (CTE) of the TiB.sub.2 -coke
mixture, or a needle coke may be used to form an anisotropic
body.
The raw materials react in situ at temperatures above 1700.degree.
C. to form a carbon-TiB.sub.2 composite according to the following
reactions:
It may also be seen that B.sub.4 C may be formed as an intermediate
step in the above.
We have found that our method produces a TiB.sub.2 -C composite in
which the TiB.sub.2 is of finer particle size and is better
dispersed throughout the structure and is made at a much lower cost
than by the addition of pure TiB.sub.2 to the dry blend of coke
particles and coke flour. It has been found easier to form
TiB.sub.2 in situ in graphite than to sinter TiB.sub.2 powder into
articles.
The composite articles produced in this manner have greatly
improved thermal shock resistance as compared to pure TiB.sub.2
articles, and greatly improved resistance to intercalation and
corrosion by the molten salt bath as compared to carbon
articles.
Other reactants may be used in place of TiO.sub.2, B.sub.2 O.sub.3
or B.sub.4 C, such as elemental Ti and B, or other Ti or B
compounds or minerals. We prefer these compounds for their ready
availability and low price, however, others may be more suitable,
based on special conditions or changes in supply and price.
When manufacturing articles in this manner, it is preferred to
impregnate the articles with a pitch and re-bake after the initial
bake cycle. Alternately, the impregnation can be accomplished after
heat treatment to 1700.degree.-3000.degree. C. Multiple
impregnations may be advantageous. In this instance the reactions
consume carbon from the coke and binder to form CO or CO.sub.2,
which escape, leaving the article highly porous, it is advantageous
to impregnate one or more times and re-bake the article before or
after heating at the high temperature cycle to densify, strengthen
and decrease porosity. If the article is an electrode or component
for a Hall cell, it may not be necessary to re-heat it to the
1700.degree.-3000.degree. C. range, after the final impregnation,
but rather to the 700.degree.-1100.degree. C. range. If the article
is to be used for an application requiring heat resistance or other
properties of graphite, it is necessary to reheat it to a high
temperature of 2000.degree.-3000.degree. C. to graphitize the coke
remaining after this last impregnation.
Another unexpected advantage is found in that articles made in this
manner may be molded or extruded, in contrast to the previously
known methods of cold pressing and sintering. Extrusion
particularly is preferred where large quantities are to be made.
Molding and extrusion methods are preferable to cold pressing and
sintering as more economical in practice, more adaptable for
production of various shapes and not requiring as complex
equipment.
Other useful sources of carbon include solvent refined coal cokes,
metallurgical coke, and charcoals.
Preferred binders are coal tar and petroleum pitches, although
other binders such as phenolic, furan and other thermosetting
resins, and organic and natural polymers may also be used. The
principal requirements are an ability to wet the dry ingredients
and have a carbon residue on baking to 700.degree.-1100.degree.
C.
DESCRIPTION OF THE INVENTION
A series of billets doped during mixing with TiB.sub.2 precursors
at 10 parts to 100 parts mix was molded and processed by heat
treatments to 2400.degree. C. and 2700.degree. C. After extensive
analyses by X-ray diffraction (XRD) and X-ray fluorescence (XRF),
it was determined that a significant portion of TiB.sub.2 was
formed from TiO.sub.2 /B.sub.2 O.sub.3 and TiO.sub.2 /B.sub.4 C
additives. Positive identification of the TiB.sub.2 was made by XRD
and distribution was observed by X-ray radiography.
Further trials resulted in the production of moldings and
extrusions containing from 3.0-75% TiB.sub.2 after heat treatment
in coke particle-flour-pitch mixes.
The mix used above was a mixture of acicular coke particles and
coke flour, bonded with about 25 parts per hundred 110.degree. C.
softening point coal tar pitch.
Various useful forms of carbon include the acicular needle type and
regular types of petroleum coke, calcined anthracite, metallurgical
coke and other selected mineral and vegetable carbons. Binders may
be coal tar or petroleum pitches, with coal tar pitches preferred
for their superior yield of carbon on coking.
The articles are formed by molding or extrusion. Cathode blocks for
Hall cells are molded or extruded, however, tubular or cylindrical
inserts for cathodes are most economically produced as
extrusions.
Baking temperatures commonly reach from about 700.degree. to
1100.degree. C., with the practice normally followed in the
examples below using a six day cycle, reaching a final temperature
on a slowly rising curve typical of those normally followed in the
electrode industry.
The acicular needle cokes, when heated to the graphitization
temperatures of 2000.degree.-3000.degree. C., will form anisotropic
graphite with coefficients of thermal expansion differing in at
least two of the three geometric axes. Regular cokes will form
isotropic graphite.
In our process, graphitization of the carbon and reaction of the
TiB.sub.2 precursors can occur simultaneously during
graphitization, forming an intimately dispersed, well bonded,
homogenous composite.
EXAMPLE 1
The following compositions were produced as modifications of a
standard carbon electrode mix.
______________________________________ Composition A B C D
______________________________________ Coke particles (acicular)
1800 g 1800 g 1800 g 1800 g Coke flour (acicular) 1200 g 1200 g
1200 g 1200 g Coal tar pitch (110.degree. C. softening point) 750 g
750 g 810 g 810 g Lubricant 15 g 15 g 15 g 15 g TiO.sub.2 160 g 223
g B.sub.2 O.sub.3 140 g B.sub.4 C 77 g TiB.sub.2 (99.5%) 300 g
Whole piece AD.sup.1, g/cc Green 1.662 1.679 1.770 1.676 Baked
1.573 1.584 1.655 1.617 Heated at 2400.degree. C. 1.425 1.393 1.494
1.498 Heated at 2700.degree. C. 1.448 1.395 1.501 1.516 XRD Scan
2400.degree. C. C C,TiB.sub.2 * C,TiB.sub.2 * C,TiB.sub.2 *
2700.degree. C. C C,TiB.sub.2 * C,TiB.sub.2 * C,TiB.sub.2 *
______________________________________ .sup.1 Apparant Density
*Weak, unidentified lines in Xray diffraction.
The compositions above were made by premilling and blending the
TiB.sub.2 or the reactants with the coke particles and coke flour
in a heated mixer, then the pitch was added, melted and the blend
mixed while hot. A larger amount of pitch was added in C and D
above to compensate for the increased surface area and binder
demand of these blends. The pieces were molded using a pressure of
2000 psi (140.6 kg/cm.sup.2) on a 33/4 in. (9.5 cm) diameter
molding, baked to about 700.degree. C., then transferred to a
graphitizing furnace, and heated to 2400.degree. or 2700.degree.
C.
Results from X-ray diffraction and X-ray radiography indicate a
significant amount of TiB.sub.2 formation from the reactants in B
and C above, at a calculated level of 7.38%.
EXAMPLE 2
The following compositions were made with higher concentrations of
TiB.sub.2 and precursors than in Example 1. The additives were
incorporated at 100 pph level in the heated coke mix before the
addition of binder. The formulations were mixed in a heated sigma
mixer, molded at 2000 psi (140.6 kg/cm.sup.2) for 5 minutes at
113.degree.-116.degree. C., and baked to about 700.degree. C., in a
six day cycle, with results as follows:
______________________________________ Composition, pbw E F G H
______________________________________ Coke particles.sup.1 60 60
60 60 Coke flour.sup.2 40 40 40 40 Coal tar pitch 25 41.7 41.7 36.7
Lubricant 0.5 0.8 0.8 0.8 TiO.sub.2 /B.sub.2 O.sub.3.sup.3 100
TiO.sub.2 /B.sub.4 C.sup.4 100 TiB.sub.2 (99.5%) 100 TiB.sub.2,
calculated % 46.8 32.2 42.2 Whole piece AD, g/cc Green 1.682 1.943
2.118 2.134 Baked 1.531 1.593 2.075 2.097 Heated-2400.degree. C.
1.450 1.104 1.605 1.974 Approx. TiB.sub.2 (XRD) % trace 3.4 34 28
Contaminants identified by XRD TiO.sub.2, TiC TiC TiC Condition
after 2400.degree. C. OK weak, weak, OK porous porous
______________________________________ .sup.1 Av. diam. 3 mm
acicular coke .sup.2 52% min. -200 mesh acicular .sup.3 In
stoichiometric ratio according to the equation TiO.sub.2 + B.sub.2
O.sub.3 + 5 C .fwdarw. TiB.sub.2 + 5 CO. .sup.4 In stoichiometric
ratio according to the equation 2 TiO.sub.2 + B.sub.4 C + C
.fwdarw. 2 TiB.sub.2 + 2 CO.sub.2.
EXAMPLE 3
Moldings were made using coke flour and TiB.sub.2 at various
percentages with results as follows, after mixing, molding and
baking as in Example 1.
______________________________________ Composition, pbw I J K L M N
______________________________________ Coke flour, isotropic 80.1
61.4 37.8 Coke flour, acicular 79.9 61.4 37.8 TiB.sub.2 (90.9%)
19.9 38.6 62.2 20.1 38.6 62.2 Coal tar pitch 38 32 27 38 32 27
Lubricant 1 1 1 1 1 1 Calc. TiB.sub.2 15 30 50 15 30 50 Whole piece
AD, g/cc Green 1.818/ 1.988/ 2.307/ 1.866/ 2.024/ 1.817 1.989 2.294
1.857 2.005 2.322 Baked 1.733/ 1.931/ 2.237/ 1.693/ 1.900/ 1.742
1.918 2.213 1.702 1.863 2.242 Impregnated, wt. % pickup.sup.1 7.4
3.6 0.6 7.8 1.4 1.6 Rebaked AD, g/cc 1.81 1.981/ 2.258 1.763/
1.927/ 2.261 1.979 1.793 1.908 Heated to 2400.degree. C., AD 1.84
2.263 2.217 TiB.sub.2 by 7 24 19 XRD, %
______________________________________ .sup.1 Impregnated with
petroleum pitch with a softening point of 115.degree.-120.degree.
C. and rebaked to about 700.degree. C.
Two moldings were made for most of the above formulations, molded
at 2000 psi (140.6 kg/cm.sup.2) for 5 minutes at die temperatures
of 115.degree.-120.degree. C.
EXAMPLE 4
Pieces were formed by extrusion of mixtures made according to the
procedure of Example 1, with the following compositions and
results.
______________________________________ Composition, parts by weight
O P ______________________________________ Isotropic coke flour
60.6 60.6 TiB.sub.2 (90.9%) 39.4 39.4 Coal tar pitch 32 32
Lubricant 1.5 1.5 TiB.sub.2, calculated % 29.5 29.5 Whole piece AD,
g/cc Green 1.962 1.973 Baked 1.891 1.902 Extrusion conditions Mud
pot .degree.C. 115-120.degree. C. 115-120.degree. C. Die
temperature, .degree.C. 110 110 Extrusion pressure (psi) 500 500
(kg/cm.sup.2) 35 35 ______________________________________
EXAMPLE 5
Moldings were made as in Example 1 with the following
compositions:
______________________________________ Composition, pbw Q R S T U
______________________________________ Coke flour, isotropic 15
52.5 71.3 50 TiB.sub.2 - 99.9%.sup.1 100 85 TiO.sub.2 35.3 21.3 15
B.sub.4 C 12.2 7.4 Borax 35 Pitch, 110.degree. C. 21 24 32 38 25
Green Whole piece AD, g/cc 3.050 2.750 2.040 1.850 1.820 Calculated
TiB.sub.2 % 88%.sup.2 74%.sup.2 31%.sup.2,3 19%.sup.2,3 41%.sup.2,4
______________________________________ .sup.1 Very high purity
TiB.sub.2, 99.9% + assay. .sup.2 Assuming 65% coke yield on coal
tar pitch after baking to 700.degree.-1100.degree. C. range. .sup.3
Assuming reactions as in Example 2 .sup.4 Assuming the reaction: 2
TiO.sub.2 + Na.sub.2 B.sub.4 O.sub.7.10 H.sub.2 O + 10 C .fwdarw. 2
TiB.sub.2 + Na.sub.2 O + 10 H.sub.2 O + 10 CO
* * * * *